In the field of optical metrology, particularly in the area of photogrammetry, practical systems are dependent upon the precison with which measurement may be made of relative locations of points on a subject under observation. It is essential, for example, that features on an aerial photograph of a land mass be precisely located with respect to established datum points in order that basic orientation parameters may be calculated in the process of constructing terrain stereo models.
Devices such as mono- and stereocomparators, which are employed to dimension coordinates of reference terrain features, and stereocompilers, which yield stereoscopic terrain views from analyses of such coordinate dimensions, rely for their utility on the accuracy with which displacement in subject photographs may be measured. These instruments are substantially similar in that they comprise means for mounting a photographic transparency for viewing with optical means while effecting relative displacement between the viewing optics and the photograph, as well as means for measuring such displacement. Of these elements the measuring means have consistently been the most critical and have presented the greatest difficulties to realizing necessary accuracies.
In their simplest form these devices consist essentially of a stationary base upon which is supported a first platform or stage constrained for movement along a first horizontal axis, the first stage supporting a second constrained for movement along the orthogonal horizontal axis, and the second stage supporting the photograph to be examined. A microscope affixed to the base and supported in a stationary position above the diapositive photograph plate includes a reticle which establishes the reference datum while the terrain features in the photo are being observed in light transmitted through the plate. Index marks on the base and first stage are employed in conjunction with respective graduated scales extending along the orthogonal displacement axes on the first and second stages to establish the coordinates of terrain features and the extent of photo displacement with respect to the reference reticle.
From these simple beginnings, photogrammetric instruments have been improved in measuring accuracy by the introduction of lead screw or other precise displacement mechanisms, and electro-mechanical measuring devices, such as angle encoders. Such features are suggested, for example, in U.S. Pat. No. 3,116,555. The problems of achieving requisite accuracy persisted, however, due to the mechanical tolerances intervening between the photograph and the ultimate measuring element.
With each linkage or interface in such a mechanical train providing a source of measuring error, efforts were made to more closely associate the displacement measuring means with the photo itself in order to eliminate the discrepancies inherent in available systems. To this end arrangements such as suggested in U.S. Pat. No. 3,330,964 provided electro-optic coordinate measuring scales which were an integral part of a single stage support by means of which the photo/scales combination could be displaced as a unit with respect to the stationary combination of viewing optics and scale-reading sensors.
Although a significant improvement, such a system suffered from the excessive offset between the viewing line of sight and the measuring elements necessitated by the requirement for an unobstructed light path over the whole area of the photographic transparency. The resulting extended moment arm between photo reference point and measuring means led to torsional displacement errors which could not be tolerated in precise photogrammetric operations. The system had the disadvantage also of requiring a working surface of sufficiently large area to accommodate the scale elements as well as the subject photograph.
Some improvements in measuring accuracy and equipment size reduction were realized with systems such as described in U.S. Pat. No. 3,729,830 which arranged the photo and biaxial scale grid generally in line with the viewing line of sight. The persistent requirement for lack of obstruction in the viewing path resulted, however, in the separation of the scale and photo elements. This in turn limited the utility of such an arrangement due to the need for maintaining critical parallelism between the photographic plate and the scale grid across distances sufficient to accommodate elements of the viewing optics in the intervening space, and for providing an unerring mechanical system for coupling the photo displacement to that of the scale sensors.
In an attempt to optimize these systems with respect to both size and stability, consideration has been given to incorporating the biaxial scale grid into the transparent photo support plate and to physically associating displaceable viewing optics with the sensors of the measuring system. In this manner the device need be only as expansive as the subject photo, since the optics would be capable of moving to any feature to be examined. Further, the displacement sensors would be closely adjacent to the line of sight, thereby substantially eliminating the disruptive mechanical offsets of earlier systems.
A major obstacle to the utility of such an arrangement remained, however; namely, the presence of the scale grid in the path of image-bearing light as a result of situating the sensor/optics couple close to the grid surface of the photo support plate. The problem which this creates arises from the fact that any previously available scale grid structure, whether amplitude or phase grating type, imposes a diffractive element in the viewing line of sight which significantly degrades the image of photographic features under examination. In addition to diffraction, these gratings often cause an attenuation of the image beam which renders the system of little practical use.
The present invention alleviates this problem by providing a phase grating type grid structure which yields an electro-optically sensible scale, yet causes no deleterious diffraction of transmitted visible light. Unlike the amplitude grating described in U.S. Pat. No. 3,768,911 or the phase grating of U.S. Pat. No. 3,482,107, the grating of this invention presents no differences in optical thickness with respect to normally-incident transmitted light and thus avoids creating diffraction of such image-bearing light. On the other hand, the incident light of a sensing system, such as shown in the noted U.S. Pat. No. 3,768,911, is sufficiently diffracted in reflection to generate the fringe pattern which forms the basis for that precise electro-optical measuring system.